In order to make this checkbox in the checkout pre-checked by default:
You need to add ng-checked="true" to the checkbox in the following code in the Payment Components template:
HTML
<input type="checkbox" name="defaultPaymentMethodId" ng-model="cc.defaultPaymentMethodId"
id="default-payment-method-credit-card" ng-checked="true"/>
<label for="default-payment-method-credit-card">
<t>Apply this payment method to all active subscriptions</t>
If you'd like to make this checkbox checked in My Account, you would need to edit the My Account Library Components and My Account Wallet Components templates and add ng-checked="true":
HTML
<div class="apply-this-method-checkbox">
<input type="checkbox" id="default-payment-method" ng-model="card.needToApplyDefaultPaymentMethod" ng-checked="true">
<label for="default-payment-method"><t>Apply this payment method to all active subscriptions</t></label>
</div>